This invention relates to a circuit arrangement for operating at least two discharge lamps, provided with
input terminals for connection to a supply voltage source,
means I coupled to the input terminals for generating a high-frequency voltage from a supply voltage delivered by the supply voltage source,
a load branch B coupled to the means I and comprising
a first branch A comprising first lamp terminals for accommodating a discharge lamp and a first inductive element L1,
a second branch C shunting the first branch A and comprising further lamp terminals for accommodating a discharge lamp and a second inductive element L2 which is magnetically coupled to the first inductive element L1 and forms an equalizer transformer together with the first inductive element L1, and
means II coupled to the means I for adjusting the light output of the discharge lamps.
Such a circuit arrangement is known from EP 0766500. The known circuit arrangement is suitable for operating two discharge lamps. The equalizer transformer achieves during lamp operation that the currents through the two discharge lamps are approximately equal. This is important especially when the circuit arrangement offers the possibility of dimming the discharge lamps, since otherwise the luminous fluxes of the discharge lamps may differ considerably in the dimmed state, which is regarded as undesirable in many applications.
The known circuit arrangement is also equipped with means for limiting the voltage over the not yet ignited discharge lamp in case one of the discharge lamps has ignited while the other discharge lamp has not yet ignited. Damage to the not yet ignited discharge lamp is thereby prevented, while at the same time a current with a comparatively high amplitude through the equalizer transformer is prevented. It is a disadvantage of the known circuit arrangement, however, that removal of one of the discharge lamps from the lamp terminals can not easily be detected by monitoring the voltage between the lamp terminals. More in particular this is true in case the discharge lamps are dimmed. Safety requirements often demand that the operation of the circuit arrangement is stopped or changed in such a way that no dangerous situation results upon removal of one of the discharge lamps. For this reason many known ballast circuits for operating a discharge lamp are equipped with a stop circuit that monitors the voltage between the lamp terminals and for instance switches the ballast circuit off in case this voltage is higher than a predetermined value during a predetermined time interval. Such a stop circuit, however, would not function properly when applied in a circuit arrangement as disclosed in EP 0766500.
An object of the invention is to provide a circuit arrangement for operating at least two discharge lamps equipped with an equalizer transformer that is equipped with effective means to maintain safety in case one of the lamps is removed from the circuit arrangement.
A circuit arrangement as mentioned in the opening paragraph is therefore characterized in that the circuit arrangement is in addition provided with means III, coupled to the equalizer transformer for comparing a voltage over the equalizer transformer with a reference voltage and for changing the operating condition of the circuit arrangement in case the voltage over the equalizer transformer is higher than the reference voltage.
When one of the discharge lamps is removed from a circuit arrangement according to the invention the voltage over the equalizer transformer changes substantially. This was found to be true for a wide range of the light output of the discharge lamps. Lamp removal or extinguishing of one of the discharge lamps therefor dependably results in the equalizer voltage becoming higher than the reference voltage so that the means III change the operating condition of the circuit arrangement.
The change in operating condition can be an interruption of the supply voltage so that the circuit arrangement stops operating.
Alternatively, in a preferred embodiment the means I comprise two switching elements that are rendered alternately conductive and non-conductive by means of a control signal generated by a control circuit and the means III are equipped with means for interrupting the control signal. In this preferred embodiment adjusting the light output of the discharge lamps can be realized by changing the frequency of the high frequency voltage by changing the frequency of the control signal. Alternatively, it is also possible to adjust the light output of the discharge lamps by adjusting the duty cycle of the control signal.
It has been found that in case the light output of the lamps is adjusted at a very low level, e.g. less than 10% of the nominal light output, the rise in the voltage over the equalizer transformer resulting from removal of one of the lamps is relatively low. As a consequence lamp removal cannot be dependably detected in case the light output of the discharge lamps is chosen very low. Circuit arrangements that can control the light output of the discharge lamps at such a low level are often equipped with means IV for supplying electrode heating currents to the electrodes of the discharge lamps. These electrode heating currents can be increased in case the discharge currents through the discharge lamps and the light output of the discharge lamps both decrease. In this way the electrodes of the discharge lamps can be maintained at approximately the same temperature for different levels of the light output and the lamp life can be lengthened. A further preferred embodiment of the invention comprises means IV for supplying electrode heating currents to the electrodes of the discharge lamps and means V for comparing the sum of the electrode heating currents with a further reference voltage and for changing the operating condition of the circuit arrangement in case the sum of the electrode heating currents is smaller than the further reference voltage. The electrode heating current has a relatively high value when the light output of the discharge lamps is at a low level. When one of the discharge lamps is removed the electrode heating current through the electrodes of that discharge lamp changes from the relatively high value to zero. This results in a big enough change in the sum of the electrode heating currents to provide a dependable detection of lamp removal. The change in operating condition effected by the means V is preferably the same as the change effected by means III.